1. Field of the Invention
The invention pertains to a range measurement system as used, in particular, as a parking aid system in a motor vehicle.
2. Description of the Art
As a vehicle parallel parks, this type of system shows the driver the presence of an obstacle and indicates the distance of this obstacle and its position with respect to the vehicle. The principle of this type of range measurement system is described, for example, in DE-OS 3,540,704. The principle of the method of operation of this type of parking aid system is as follows: an electronic control unit (ECU) is connected to bidirectional data lines that have individual ultrasonic sensors (ULS) which are distributed over the periphery of the vehicle. The individual ultrasonic sensors are controlled as a function of a sequence stored in the electronic control unit (ECU). The ultrasonic sensors then emit a pulse-shaped wave train that is reflected, for example, by means of an existing obstacle. The transmitted waves, are sound waves in the ultrasonic range. When the sound waves are reflected by means of an obstacle, they are then received by means of the ultrasonic sensors, which operate not only as transmitters, but also as receivers. The individual ultrasonic sensor now produce pulses on the data line that correspond chronologically with the received echo, as long as their amplitude exceeds a threshold set in the ultrasonic sensor. With respect to the pulse, the level may be identified as digital, insofar as the existence of a pulse is simultaneously a statement to the effect that the pulse exceeds a certain threshold value (threshold is exceeded or not exceeded). Conversely, the chronological position of the pulse indicates analog information, namely, the range of the obstacle from the receiving ultrasonic sensor. This analog information results from the travel time response of the ultrasonic pulse. Thus, the greater the range to the obstacle, the later the echo arrives at the receiving sensor. The electronic control unit (ECU) computes the distance to the obstacle from the time between the transmission process until the appearance of the reflected wave, keeping in mind the speed of sound.
Parking aid systems of today usually consist of an electronic control unit and, as already explained above, several ultrasonic sensors. Each ultrasonic sensor (ULS) possesses a separate data line, since, in order to improve evaluation, echo information from several ultrasonic sensors is required at a certain instant. Thus, the evaluation of several ULS on the basis of one transmitted sound wave permits more precise conclusions to be made about the position of and the range of the obstacle. It has also already been proposed to arrange the threshold within the ultrasonic sensor so that the exceeding of the threshold triggers the emission of a pulse on the data line. It also is possible to change the threshold within the ultrasonic sensor, such that, for example, several pulses, which are evaluated with changing thresholds, could be emitted in succession from one ultrasonic sensor. In reality, this means that individual echoes are received with differing sensitivity. Also, the sensitivity of a sensor may be changed across the receiving time of a pulse echo, such that certain fixed, preset obstacles such as, for example, a trailer coupling, projecting luggage and the like, may be blanked out. The same applies to reflection from the pavement itself, which appears at a certain range. However, the electronic control unit (ECU) obtains, on the basis of the digital data line, only the information at each instant of whether the amplitude level exceeds the respective threshold or remains below the threshold. As also described, for example, in DE-OS 3,513,270, three states are arranged in succession in the chronological progression of a measurement cycle associated with a single ultrasonic sensor. The ECU produces a short transmission pulse (&lt;1 ms) . Then the receiving ultrasonic sensor transmits the information to the ECU (approximately 10 ms). Finally, a "recovery time" is provided, in which noise echoes and reflections can decay (approximately 10 ms).